The invention relates to a magnetic bearing where a shaft which can rotate is borne magnetically within a stator. The magnetic bearing is intended to have the following features:
a first bearing part is rigidly connected to the shaft and is surrounded by a second bearing part, which is associated with the stator, forming a bearing gap between these bearing parts,
the first bearing part contains a magnet arrangement with permanently magnetic elements,
the second bearing contains a superconducting arrangement with a high-Tc superconductor material, with magnetic bearing forces being produced between the superconducting arrangement and the permanently magnetic elements of the magnet arrangement, and
a cooling apparatus is provided for cooling the superconductor material of the superconducting arrangement to an operating temperature below the critical temperature of the superconductor material.
A magnetic bearing such as this is disclosed in DE 44 36 831 C2.
Magnetic bearings allow moving parts to be provided with bearings which make no contact and are therefore free of wear. They require no lubricants and can be constructed to have low friction. In this case, a body which can rotate (rotating body) can be hermetically sealed, that is to say in a vacuum-tight manner, from the outer area surrounding it.
Known magnetic bearings use magnetic forces between stationary electromagnets on a stator and ferromagnetic elements, which are on a rotor body and rotate with it. With this type of bearing, the magnetic forces are always attractive. In consequence, it is in principle impossible to achieve a bearing which is inherently stable in all three spatial directions (see xe2x80x9cEamshaw""s Theoremxe2x80x9d in xe2x80x9cTrans. Cambridge Phil. Soc.xe2x80x9d, Vol. 7, 1842, pages 97 to 120). Magnetic bearings such as these therefore require active bearing control, which uses position sensors and a control loop to control the currents and the supporting magnets and to counteract any discrepancies of the rotor body from a nominal position. The control process, which needs to have a plurality of channels for this purpose, requires complex power electronics. In addition, a mechanical emergency bearing must be provided as a precaution against sudden failure of the control loop. Corresponding magnetic bearings are used, for example, in turbo-molecular pumps, ultra-centrifuges, high-speed spindles for machine tools, and X-ray tubes with rotating anodes; use for motors, generators, turbines and compressors is likewise known.
In principle, superconductors allow a new type of magnetic bearing: one of the bearing parts is in this case formed with permanently magnetic elements which induce shielding currents in the event of a position change, as a consequence of field changes in the superconductor material of a further, second bearing part which surrounds the first bearing part with a gap. The forces which result from this may be repulsive or attractive, but are directed such that they counteract the deflection from a nominal position. In contrast to known magnetic bearings, it is possible in this case to achieve an inherently stable bearing (see, for example, xe2x80x9cAppl. Phys. Lett.xe2x80x9d, Vol. 53, No. 16, 1988, pages 1554 to 1556). In contrast to known magnetic bearings, there is no need here for any complex control system that is susceptible to defects; however, a cooling apparatus must be provided in order to cool the superconductor material to an operating temperature below the critical temperature of the superconductor material.
Appropriate superconducting bearing parts for magnetic bearings such as these may be one of the first fields of use for the metal-oxide high-Tc superconductor materials which have been known since 1987, such as those based on the Y-Ba-Cu-O material system, which can be cooled to an operating temperature of about 77 K using liquid nitrogen.
Use of appropriate high-Tc superconductor material is envisaged for the magnetic bearing which is disclosed in the DE-C2 document cited initially. The magnetic bearing contains a large number of permanently magnetic elements which are in the form of annular discs and are located one behind the other in the axial direction on a rotor shaft. These elements are polarized such that the polarization alternates when seen in the axial direction of the shaft. Comparatively thin ferromagnetic intermediate elements are arranged in each case between adjacent elements. These intermediate elements primarily have the task of magnetically concentrating the magnetic lines of force of adjacent permanently magnetic elements, so that a particularly high magnetic field strength is produced on the side of each intermediate element which faces the bearing gap. This bearing part of the rotor body, together with its magnet arrangement composed of permanently magnetic elements, is surrounded by a fixed-position bearing part of a stator. This bearing part contains a superconducting arrangement with a high-Tc superconductor material such as Yba2Cu3Ox, with the abovementioned magnetic bearing forces being produced between the superconducting arrangement and the permanently magnetic elements of the magnetic arrangement. The superconductor material of the conductor arrangement is kept at about 77 K by liquid nitrogen (LN2). For this purpose, cooling channels through which this coolant is passed are provided on the outside of the superconducting arrangement.
In the case of magnetic bearings in which parts which need to be cryogenically cooled are adjacent to the bearing gap, one problem that can occur is that environmental air can reach the cold components through the bearing gap, with the moisture in the air freezing there. Corresponding icing can lead to functional restrictions or damage to the bearing. In the case of the magnetic bearing which is disclosed in the abovementioned DE-C2 document, such icing of the bearing gap can be avoided by emitting vaporizing nitrogen. The necessary cooling power for the bearing is in this case from a few watts up to the order of magnitude of 10 W at 50 to 80 K. However, if other cooling techniques than those used for the known magnetic bearings are envisaged, especially using so-called cryogenic coolers with only indirect cooling, there is no corresponding capability to avoid the risk of icing in the bearing gap, since no vaporizing coolant gas is then available.
One possible object of the present invention is therefore to refine the magnetic bearing having the features mentioned initially, such that such risk of bearing icing is minimized irrespective of the chosen cooling technique, and such that the sealing complexity can be kept low.
This object may be achieved in that, in the case of the magnetic bearing having the features mentioned initially, the superconducting arrangement and the magnetic arrangement are also jointly surrounded by at least one isolation area, and in that an additional area is provided, which is separated from the at least one isolation area and comprises the bearing gap and subareas which extend on side end faces of the superconducting arrangement and of the magnet arrangement radially as far as the shaft and are sealed there with respect to the shaft.
The advantages which are associated with this embodiment of the magnetic bearing are, in particular, that the complexity for sealing the additional area from the parts which can rotate can be kept low. This is because the seal uses the smallest possible diameter, so that the circumferential speed of the parts of the seal which also rotate is minimized. This makes it simpler for the seal to operate, and correspondingly lengthens its life. The simplified sealing, which may thus also be designed to be effective, of the additional area also results in the risk of ingress of gases which can freeze at least largely being avoided.
The additional area of the magnetic bearing can thus be evacuated in a simple manner. This advantageously allows friction losses to be reduced. In the event of any leakage of the sealant on the shaft, a small amount of air could admittedly in theory enter, however, severe icing is counteracted by the fact that, in this case, even a defective seal still provides a major impediment to the exchange of air, especially because the corresponding flow cross sections of the side subareas can be kept small. The magnetic bearing therefore has good emergency running characteristics.
Instead of this, it is particularly advantageous to fill the additional area with a dry barrier gas. Any gas or gas mixture which has no components that freeze at the operating temperature in the area of the bearing gap is suitable for use as the dry barrier gas. Appropriate barrier gases can be chosen from the group of helium, neon, argon or nitrogen, with a gas mixture having at least one of these gases also being suitable. If the additional area is filled with a gas, then the temperature decreases from the hot shaft toward the cold bearing gap in the side subareas, which can advantageously be formed with a small cross section, without any thermal losses occurring due to convection. This is because convection is avoided by the hot end of the side subareas filled with gas whose density is less being located closer to the shaft. The centrifugal force then results in stable layering during rotation. Furthermore, it should be regarded as being particularly advantageous that operation is also possible with slightly fluctuating pressures in the gas area, so that gas losses due to leakages, for example in the sealing area, are tolerable within wide limits. Depending on the requirement for the applications, the gas pressure may be below 1 bar, around 1 bar or more than this, so that especially in the latter case entry of moist air with ice formation in the cold area is reliably prevented.
In order to provide effective thermal isolation for those parts of the magnetic bearing which need to be cooled, it is possible, in particular, to evacuate the at least one isolation area. Instead, or preferably in addition to this, this area can also at least partially be filled with at least one isolation means, which is known per se.
It should also be regarded as particularly advantageous that the cooling apparatus for the magnetic bearing has at least one cryogenic cooler with at least one cold head. This cold head is then thermally coupled to the superconducting arrangement, preferably via at least one thermally conductive body, in order to provide indirect cooling for the superconducting arrangement. The use of such a cryogenic cooler has the advantage that the cooling power is available virtually at the push of a button, and there is no need to handle cryogenic liquids. In this case, indirect cooling by thermal conduction to the cold head is sufficient for effective cooling of the known high-Tc superconductor materials. If a cryogenic cooler is used, it is admittedly not possible to prevent icing of the bearing gap by the emergence of vaporizing cooling gas such as nitrogen. Furthermore, thermal isolation of the superconductor material in the bearing gap would result in considerably enlargement of the bearing gap and would thus also result in a corresponding drastic reduction in the supporting force and stiffness of the bearing. For good operation, the bearing gap should, however, be as small as possible and, for example, should be in the order of magnitude of 1 mm. If the bearing were to be placed completely in an isolating vacuum vessel for this purpose, then this in principle would have to be sealed from the rotating shaft by two hermetic seals. This would in fact have the disadvantage that the vacuum would break down in the event of leakage, correspondingly interfering with the operation of the bearing and of the machine parts being borne.
In this case, the permanently magnetic elements would then be cooled down slowly essentially by heat radiation to an intermediate temperature between the operating temperature of the superconductor material and the outside temperature. The configuration of the special additional area, which is separated from the isolation area which jointly surrounds the superconducting arrangement and the magnetic arrangement, advantageously, however, allows the corresponding sealing problems to be overcome sincexe2x80x94as already statedxe2x80x94the sealing of the additional area is subject to considerably less stringent requirements. Even indirect cooling can therefore also be used without any problems.